The present invention relates to a data transmission system, and more particularly to a data transmission system for utility metering.
The development and commercialization of various utility products, such as electricity, water, and gas have contributed to dramatic advances in living standards. In allocating usage bills, utility companies typically gauge consumption using meters and bill their customers accordingly. Traditionally, at the end of a reporting period, a utility employee physically inspects and records each customer's meter readout dials which reflect usage. The recorded data is eventually entered into an accounting system for billing purposes. This process is labor intensive and duplicative. Moreover, the meter reading process may be disrupted by unplanned nuisances such as dogs and inclement weather. Further, this process cannot provide time-of-day metering so that the utility company can charge for the utility product as a function of load factors.
To overcome these inefficiencies, remote meter reading systems have been developed which automatically capture consumption data from the field. In situations which cannot afford dedicated lines or connections to the plain old telephone service (POTS), wireless meter reading systems have been deployed. Typically, such a wireless meter reading system includes a base station which transmits on one frequency to a remote station, which in turn may relay the transmission to other remote stations. The base station also receives data from the remote stations on the same or related frequency. The wireless transmission of data between the base and remote stations is determined by the licensing rules of a government regulatory authority such as the Federal Communications Commission (FCC) in the United States or the Radio-Communications Authority (RA) in the United Kingdom (UK).
Generally, the authority grants licenses to operate radio transmitters that have to operate within a limited frequency spectrum. For instance, in the UK, a spectrum between 183.5 MHZ and 184.5 MHZ is reserved for metering applications. The band is in turn divided into eight 25 kHz channels and four 200 kHz channels. Similar frequency allocations are also enforced in various other countries which reserve a band of frequencies for various applications such as meter data collection.
Due to the limited frequency spectrum, data transmission needs to be within a narrow range such as within about 100 kHz of a predesignated transmission frequency. Since the wireless meter reading system deploys many more transmitters than receivers in forwarding usage statistics to a utility company's central location, each transmitter needs to be made as economically as possible. Typically, the most expensive component in the transmitter is a quartz crystal resonator which controls the transmission frequency.
The resonators use crystals made of quartz in the frequency generation process. Due to the cost of the crystal, it is desirable to use the least possible expensive grade of crystal. However, low grade crystals tend to be more sensitive to ambient and operating temperature variations. Further, over time, the frequency generated by low grade crystals tend to drift. As such, the use of low grade, inexpensive crystals tends to reduce the accuracy of the transmitter's operating frequency. Further, other components associated with the frequency generation process are also subject to aging. For instance, as capacitors age, variations in their capacitive values may cause a frequency variation in a .+-.10% range.
Thus, the use of inexpensive components may cause the transmission frequency to vary out of alignment during operation as the components heat up. Further, over time, the transmission frequency may drift out of alignment due to aging. If the frequency variations are significant, the components associated with the frequency generation process need to be replaced or aligned so that the base station and the remote stations can communicate with each other in the designated frequency range. Such replacement or alignment operations are cost prohibitive and for many applications, not practicable. Alternatively, higher grade, but more expensive crystals could be used.